1. Field of the Invention
The invention is directed to the reheating of recycle gas used in indirect retorting of oil shale through the utilization of residual carbon in oil depleted shale and, more particularly, it is directed to a process and an apparatus for improving the thermal efficiency of an indirect oil shale retorting process by burning most of the organic carbon remaining in the oil depleted shale in a separate vertical shaft furnace with the temperature of combustion moderated by the recycling of cooled spent shale.
2. Description of the Prior Art
While there are several methods known by which oil can be recovered from oil bearing shale, it has been found that the use of a traveling grate as the principal processing equipment is preferred. This equipment can be scaled up to a very large commercial size with predictable results and therefore traveling grates, both straight and circular, present a technically and economically feasible method of producing oil from oil shale. The traveling grate may be operated in either of two modes, a direct shale heating mode or an indirect shale heating mode. In the direct mode, air is injected directly into the bed of shale and heat is generated in situ by the combustion of gas and some carbon, and possibly some of the oil from the shale. The exhaust gas from the direct heating process is not of pipeline quality. In addition, most of the organic carbon remains unburned. In the indirect mode of heating the shale, the process gas is heated prior to its contact with the shale and is recirculated through the shale to effect oil liberation and to recover the liberated oil.
In both of these methods, most of the organic carbon produced in the decomposition of the kerogen remains in the shale. In the direct method, the carbon on the surface of the shale is burned but carbon depletion does not extend very far into the shale. This occurs because the burning of the interior carbon depends on the inward diffusion of oxygen in competition with the outward diffusion of carbon monoxide. This is a relatively slow process when compared to the decomposition of shale kerogen. The unburned carbon is left in the shale and the thermal requirements of the retorting process are provided by burning the gas produced in the kerogen decomposition. In the indirect method, none of the carbon is burned and all of the thermal requirements of the retorting process are met by the externally heated process gas.
While the direct heating method has the advantage of a higher oil production rate per unit area of the grate, the indirect heating method renders a higher oil yield and a product gas that is of pipeline quality. The instant invention is directed to a method for the improvement of the thermal efficiency of the indirect heating process.
It has been suggested that the carbon content of spent shale be recovered and utilized in the production of thermal energy. For example, U.S. Pat. No. 2,434,815 teaches a method for the production of steam by combusting the carbon in spent shale in a vertical retort. In U.S. Pat. No. 3,617,466, off-gas from a retort is burned externally to the retort and combustion gases recycled to control the temperature in the retort and prevent clinker formation, U.S. Pat. No. 4,218,304 teaches the combusting of recycle gas outside a retort and the returning of the hot gaseous products to the retort in order to control carbonate decomposition, coking or carbonization of the gas during heating.
In U.S. Pat. No. 4,297,201, the residual carbon in indirectly retorted oil shale is burned out as the retorted shale descends through a vertical kiln separate from the retort vessel. An inert gas is circulated in a closed loop through the kiln to control the temperature and absorb the heat of combustion, and through a heat exchanger which transfers the heat to the process gas for indirect retorting of additional oil shale.